Thermal spraying is utilized in numerous industries to apply protective coatings to metal substrates. More recently, thermal spray methods have been the focus of attention for the fabrication of high-tech composite materials as coatings and as freestanding near net structures. By heating and accelerating particles of one or more materials to form a high-energy particle stream, thermal spraying provides a method by which metal powders and the like may be rapidly deposited on a target. While a number of parameters dictate the composition and microstructure of the sprayed coating or article, the velocity of the particles as they impact the target is an important factor in determining the density and uniformity of the deposit.
One prior art deposition technique known as "plasma spraying" employs a high-velocity gas plasma to spray a powdered or particulate material onto a substrate. To form the plasma, a gas is flowed through an electric arc in the nozzle of a spray gun, causing the gas to ionize into a plasma stream. The plasma stream is at an extremely high temperature, often exceeding 10,000 degrees C. The material to be sprayed, typically particles from about 20 to 100 microns, are entrained in the plasma and may reach a velocity exceeding the speed of sound. While plasma spraying produces high-density coatings, it is a complex procedure which requires expensive equipment and considerable skill for proper application.
A combustion flame has also been used to spray powdered metals and other materials onto a substrate. A mixture of a fuel gas such as acetylene and an oxygen-containing gas are flowed through a nozzle and then ignited at the nozzle tip. The material to be sprayed is metered into the flame where it is heated and propelled to the surface of the target. The feedstock may comprise a metal rod which is passed axially into the center of the flame front or, alternatively, the rod may be fed tangentially into the flame. Similarly, a metal powder may be injected axially into the flame front by means of a carrier gas. Many combustion flame spray guns utilize a gravity feed mechanism by which a powdered material is simply dropped into the flame front. Conventional combustion flame spraying, however, is typically a low-velocity operation in the subsonic range and usually produces coatings which have a high degree of porosity.
In another spraying technique, an electric arc is generated in an arc zone between two consumable wire electrodes. As the electrodes melt, the arc is maintained by continuously feeding the electrodes into the arc zone. The molten metal at the electrode tips is atomized by a blast of compressed gas. The atomized metal is then propelled by the gas jet to a substrate, forming a deposit. Conventional electric arc thermal-sprayed coatings are generally dense and reasonably free of oxides, however the process is restricted to feedstock materials which are electrically conductive and available in wire or rod form which is unacceptable in some applications.
More recently, a modification of combustion flame spraying has produced high-density articles which exhibit metallurgical and physical properties that are superior to those produced using conventional flame spraying techniques. Commonly referred to as "supersonic" flame spray guns, these devices generally include an internal combustion chamber in which a mixture of a fuel gas, such as propylene or hydrogen, and an oxygen-containing gas is combusted. The expanding, high-temperature combustion gases are forced through a spray nozzle where they achieve supersonic velocities. A feedstock, such as a metal powder, is then fed into the high-velocity flame jet to produce a high-temperature, high-velocity particle stream. The velocities of the entrained particles produce coatings having higher densities than those produced by other subsonic combustion flame methods. Examples of these devices are shown in U.S. Pat. Nos. 4,342,551, 4,643,611 and 4,370,538 to Browning and U.S. Pat. No. 4,711,627 to Oeschale, et al..
Another flame spray apparatus is described in U.S. Pat. No. 2,861,900 to Smith, et al. Therein, a fluid combustible mixture is ignited in a barrel or nozzle element which comprises a confined space that is unconstricted from inlet to outlet. A feedstock, such as a metal powder, is introduced axially into the unconstricted barrel through which it is propelled to a target. The axial bore of the injector nozzle is utilized to convey both the fuel gas and the feedstock. Thus, feedstock is entrained in the fuel gas prior to combustion. During combustion, particle trajectories acquire radial components which may cause heated feedstock particles near the barrel wall to strike and accumulate on the wall surfaces. In addition, the effect of this particle motion is enhanced due to the large distance between the particle injection site and the combustion zone. This radial velocity also reduces the average velocity of the particles. As will be more fully explained, the present invention overcomes these limitations and provides numerous other advantages by providing a supersonic flame spray apparatus in which a steady-state continuous detonation reaction is created that produces an axial, collimated flow of particles and which allows indepentent regulation of the particle injection rate and the fuel gas flow rate.
Prior art thermal spray methods have been used to form composite materials by simultaneously spraying two or more distinct materials. Ceramic-ceramic composites, and ceramic-metal composites known as "cermets" or "metal-matrix composites," have been formed as coatings and as freestanding, near net shape articles by techniques other than thermal spray processes. Materials may also be fabricated by forming a first particle stream using one spray gun and then combining the first stream with a particle stream from another gun to form a combined spray at the target surface.
A method of forming a protective coating in this manner is disclosed in U.S. Pat. No. 3,947,607 to Gazzard, et al. The use of an electric arc gun and a separate oxygen/combustion gas-metalizing gun to form a combined spray deposit is briefly described. However, the coatings formed using twin spray guns do not have superior properties. In addition, the use of two separate spray guns to form composite coatings is difficult and unwieldly. It would therefore be desirable to provide a single spray gun which could be used to form composite materials such as metal-matrix composites and which achieves the benefits of supersonic flame spraying and electric arc spraying without their disadvantages. The present invention achieves these goals by providing a supersonic flame spray system in which a high-energy particle stream of a first material atomizes a molten second material to form a composite particle stream.